Conversion between GMT and local time

In parts of the world where daylight saving time is used, users must be aware of issues arising from conversion between GMT and local times. Where daylight saving time is never used (for example, Arizona) there is no problem converting from GMT to local time and back again since there is a one-to-one correlation between GMT and Local times. The conversion problems arise from the fact that changes to or from daylight saving time affect the length of a day. In the case of changing to daylight saving time, an hour is skipped altogether in local time, whereas the change back from daylight saving time leads to certain local times occurring twice.

If you wish to create a report that shows one day's worth of data for your plant, you could do so using the Configure Calculated Data Function worksheet function dialog, supplying a start time and end time exactly one day apart. These times would most likely be specified in cells in the worksheet with an array formula referring to them by cell address. Furthermore, the end time is likely to be expressed by a formula such as StartTime + "24:00:00". Excel ignores daylight saving time when calculating such formulas, so the end time would show as being the same hour, exactly one day later than the start time. For 363 days in the year (except for leap years), this formula would return 24 hours worth of data; but for one day each year there would be 23 hours worth of data and for another day there would be 25 hours worth of data.

You need to be aware that this will occur if you are using local time, and design your worksheets accordingly. That is, allow extra space in the table to accommodate the extra data as daylight saving time finishes; the table will be shorter as daylight saving time begins.

A similar, though potentially more serious, problem occurs where you wish to find out data relating to an occurrence around the time that daylight saving time starts.

Consider a plant in a U.S. state that operates under Central Time. At 2:00 a.m. on October 31, 2004 daylight saving time ends (Central Daylight Time finishes and Central Standard Time is used). The local time thus goes from 01:59:59 to 01:00:00. This is the second time it has been 01:00:00 that day. If anything goes wrong in the plant during the next hour, it will not be possible to view that data in DeltaV Reporter if local times are used. This is because, in converting to GMT, the first occurrence of times between 01:00:00 and 01:59:59 on October 31, 2004 is assumed. You would need to choose to use either GMT or GMT Offset when dealing with such issues.